The invention relates to a control key of the type comprising a digital-actuated push-button, adapted to move with respect to a fixed support between a rest position and a work position in which, for example, it actuates the movable element of an electrical switch to close it. Generally, this type of keys comprises a return member for returning the push-button to its rest position.
Said return member can be, in known manner, of the spring type. One of the disadvantages of this spring type return is due to the characteristic of the spring. For example, for a given stroke of the push-button, the return force of said button which is opposed to the pressing-in increases with the stroke. The pressing-in force must therefore always be greater than the maximum return force, i.e. than the force opposed by the spring at end-of-stroke of the push-button, in order to ensure that the said push-button has reached its work position. It can happen that the pressure exerted on the push-button is too small but yet causes a displacement of the latter without this displacement being sufficient to have reached the work position. Then, the expected result is not obtained although the operator had the feeling to have correctly manoeuvred the key. Moreover, such a device which is designed to be actuated repeatedly has its characteristics altering with time, wear, and ageing of the spring, etc.
To overcome such disadvantages, magnetic return keys have been proposed. The advantage of the magnetic return force is that it is of maximum intensity when the air gap is minimum. Thus, to move the movable part away from the fixed part, it is necessary from the start to overcome said maximum force which thereafter decreases very rapidly. Then, once the key is pressed-in, it is a certainty that the actuating force will be sufficient to make the said key reach its work position. The actuation of such a key is therefore much more reliable, since when pressing the finger on the key the operator is sure of reaching the work position. Moreover, said magnetic return means suffer virtually no deterioration with time.
But, although the principle of the magnetic return is an advantageous solution with this type of device, its production raises a number of problems. One of these problems is found in the characteristics proper of the magnetic return. Indeed, the resistance to the pressing-in of the key is imposed, this entailing that the maximum attractive force of the magnet with no air gap is dictated by that resistance. Moreover, the stroke of the key is also dictated by the technology of the controlled member and cannot be selected freely. Because of these strains, it is noted that the magnetic force for returning the key from its work position to its rest position is very small and cannot be sufficient to overcome the weight of the key increased with a few random forces of friction.
It is the object of the present invention to overcome this disadvantage by proposing a magnetic return control key fulfilling all the requirements of this type of product, using only means that are simple to produce and to assemble, and suitable for highly automated production.